High energy density storage of methane in light hydrocarbon solutions

ABSTRACT

Method for storing methane in light hydrocarbons at moderate temperatures and pressures such that the resulting solution has a high energy density. The method includes combining methane at a 50 to 80 mole percent concentration with a light hydrocarbon, such as butane, propane or liquid petroleum gas (LPG). Then the resulting solution is maintained at a temperature of between -1° C. and 38° C. and a pressure of between 8 and 14 MPa. Under these parameters, the solution has an energy density which is 40 to 67 percent that of gasoline. A motor vehicle fuel storage apparatus for the method includes an insulated storage tank containing the 50 to 80 mole percent methane solution and a cooling mechanism for maintaining the methane solution between -1° C. and 38° C. The cooling mechanism may be an arrangement of expansion valves, or connection to the air conditioning system of the vehicle, or both.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to liquid fuel solutions ofhydrocarbons and particularly to the storage of natural gas, primarilyfor vehicular use, in liquid solution with other hydrocarbons.

2. Description of Related Art

Composed essentially of methane, natural gas (NG) is an importantcompetitor in the field of alternative fuels. NG is economic, and itoffers a greater reduction in CO, NO_(x) and non-methane hydrocarbon(NMHC) emissions. However, the on-board storage limitations of NG are asevere drawback to the use of NG as a vehicular fuel.

Conventional storage techniques for NG involve compression andliquefaction methods. The former is termed compressed natural gas (CNG)while the latter is known as liquefied natural gas (LNG). CNG requiresbulky, high-pressure vessels to store a quantity of NG that deliversabout one-sixth the range of an equal amount of gasoline under normaloperating conditions. LNG provides nearly two-thirds the range of acomparable volume of gasoline, but requires cryogenic processingequipment and cryogenic storage.

In addition to the on-board storage problems of CNG and LNG, thelogistics of the preparation and delivery of these fuels poseinconveniences. CNG at 21 MPa requires a delivery system operating abovethe storage pressure to ensure that a full charge is obtained.Therefore, a compression plant must be at or very near the vehiclerefueling site.

LNG is liquified through a cryogenic procedure, and then delivered to astaging area before being dispensed to vehicles. This method requires aheavily insulated storage vessel to stage the fuel and a similarlyinsulated tank on board the vehicle.

In view of the drawbacks associated with CNG and LNG storage techniques,it is highly desirable to devise a safe, economical and convenient wayto store NG for use as a vehicular fuel. A practical natural gas storagesystem should be characterized by (1) moderate storage pressures, sothat high-pressure vessels like those required for storage of CNG areunnecessary; and (2) moderate storage temperatures, so that cryogenicequipment needed for storage of LNG is not a requirement.

SUMMARY OF THE INVENTION

The present invention is a method for storing natural gas at moderatetemperatures and pressures in solutions having high energy densities.The method includes providing a storage tank, introducing a mixture ofmethane and at least one other light hydrocarbon into the storage tank,and maintaining the mixture at moderate temperatures and pressures suchthat the mixture has an energy density greater than about 11,000 MJ/m³.

For vehicular use, an apparatus constructed in accordance with thepresent invention includes a vehicular storage tank containing a fuelmixture of methane and at least one other light hydrocarbon. Optionally,a cooling system to maintain the fuel mixture at a sub-ambienttemperature may be provided.

One object of the present invention is to provide a mixture of methaneand at least one other light hydrocarbon at a moderate temperature andpressure such that the mixture has an energy density greater thancompressed natural gas (CNG) at a pressure of 24.2 MPa.

A second object of the present invention is to provide a mixture ofmethane and at least one other light hydrocarbon at a moderatetemperature and pressure such that the mixture has an energy density inthe range of propane at 1.4 MPa, i.e in the range of 23,500 MJ/m³.

A third object of the present invention is to provide a vehicle storagetank which can be filled with a mixture of methane and at least oneother light hydrocarbon at a moderate temperature and pressure fromconventional fueling stations.

A fourth object of the present invention is to provide a vehicle storagetank which is capable of maintaining a mixture of methane and at leastone other light hydrocarbon at a moderate temperature and pressure atambient air temperatures of about 38° C. as the mixture is depleted fromthe vehicle storage tank.

A fifth object of the present invention is to provide a high energydensity fuel mixture for pipeline transmission. By providing a highenergy density fuel mixture, the pipeline pressure can be decreasedwithout reducing the amount of energy being sent through the pipeline.Alternatively, the same pipeline pressure may be used to send a greateramount of energy through the pipeline because of the high energy densityof the fuel mixture.

A sixth object of the present invention is to provide a high energydensity fuel mixture for use in electric generating stations, emergencygenerators or back-up generators. Some electric power plants use fueloil as a back-up or alternate fuel. Because of heating requirements inthe winter, fuel oil may be expensive and in short supply. The presentinvention provides storage of a high energy density methane fuel mixturewhich may be used as an alternate fuel for electric generating units.Moreover, cold ambient temperatures allow increased volumes of methanein the fuel mixture to achieve even higher energy densities. Thus, themethane mixtures disclosed herein may be substituted for any applicationemploying fuel oil or the like and methane mixtures burn more cleanlythan fuel oil.

Other objects, features and advantages of the present invention areapparent from the following detailed description when read inconjunction with the accompanying drawings and appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph of bubble pressure versus bubble temperature formethane and n-butane solutions between 50 mole percent methane and 80mole percent methane.

FIG. 2 is a graph of bubble pressure versus bubble temperature formethane and propane solutions between 50 mole percent methane and 70mole percent methane.

FIG. 3 is a graph of bubble pressure versus bubble temperature formethane and LPG solutions between 50 mole percent methane and 80 molepercent methane.

FIG. 4 is a graph of density versus bubble temperature for methane andn-butane solutions between 50 mole percent and 80 mole percent methane.

FIG. 5 is a graph of density versus bubble temperature for methane andpropane solutions between 50 mole percent and 70 mole percent methane.

FIG. 6 is a graph of density versus bubble temperature for methane andLPG solutions between 50 mole percent and 80 mole percent methane.

FIG. 7 is a graphical comparison of the amount of methane stored inn-butane with the amount of methane stored in CNG (21 MPa). Themethane/n-butane solutions are between 50 mole methane and 80 molemethane in a range of temperatures and bubble pressures.

FIG. 8 is a graphical comparison of the amount of methane stored inpropane with the amount of methane stored in CNG (21 MPa). Themethane/propane solutions are between 50 mole methane and 70 molemethane in a range of temperatures and bubble pressures.

FIG. 9 is a graphical comparison of the amount of methane stored in LPGwith the amount of methane stored in CNG (21 MPa). The methane/LPGsolutions are between 50 mole methane and 80 mole methane in a range oftemperatures and bubble pressures.

FIG. 10 is a graph of energy density versus bubble temperature formethane in n-butane solutions between 50 mole percent methane and 80mole percent methane.

FIG. 11 is a graph of energy density versus bubble temperature formethane in propane solutions between 50 mole percent methane and 70 molepercent methane.

FIG. 12 is a graph of energy density versus bubble temperature formethane in LPG solutions between 50 mole percent methane and 80 molepercent methane.

FIG. 13 is a partly diagrammatical, partly sectional view of a vehiclefuel storage system for methane solutions in n-butane, propane or LPG.

FIG. 14 is a lateral cross-section of the vehicle fuel storage systemshown in FIG. 13.

FIG. 15 is a graph of duty absorbed by flashing fuel for a 70 molepercent methane in butane solution and a 60 mole percent methane inpropane solution.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Quantification of mixture properties such as density, enthalpy, entropy,heat capacities, etc. are usually not readily obtained from tabulateddata, but must be determined by calculation. The assumption of an idealsolution allows one to calculate the properties of a mixture as thesummation of the individual components in solution. However, theproperties of a real solution are not necessarily ideal because, uponmixing, the individual components of real solutions may undergoadditional interaction characteristics which affect fluid properties.Nevertheless, solution properties can be estimated through the use of anequation of state that accounts for the solution's deviation from theideal.

The equation of state used herein for solutions of methane in lighthydrocarbons is the Benedict-Webb-Rubin-Starling (BWRS) equation, showbelow. In this equation, the pressure P is a function of the temperatureT and the molar density r. The other terms (A₀, B₀, C₀, D₀, E₀, a, b, c,d, a, g) are mixture parameters for the specific interactions of thecomponents in the mixture of interest. ##EQU1## This equation is verygood for gases and light hydrocarbons because it accounts for binaryinteractions of these types of compounds very well. The usefulness ofthis equation is that it extends into both liquid and vapor phasecalculations with very good accuracy. Thus, the properties of solutionsin either the gaseous or liquid state can be determined based on thetemperature and pressure conditions.

The utility of the BWRS equation in vapor liquid equilibriumcalculations is in predicting the bubble conditions of solutions. Theseconditions are the temperature and pressure at which a mixture of aspecific composition is saturated, i.e. the liquid is on the verge ofvaporizing.

The determination of the bubble point for solutions of NG in otherhydrocarbons yields insight into the maximum amount of NG (as a functionof the mole fraction of methane) that can be stored in liquid phase at agiven temperature and pressure.

The solvents used for this analysis were n-butane, propane and anarbitrary LPG mixture, which is shown below:

    ______________________________________    Component     Mole Percent    ______________________________________    Ethane        0.02    Propane       36.23    Isobutane     33.63    n-Butane      29.58    Isopentane    0.46    n-Pentane     0.08    ______________________________________

Heavier components such as C₆ and above were excluded from the analysis.While high mole fractions of methane are obtainable in the heavierhydrocarbons, the relative volume of methane which can be stored is farless than in the lighter hydrocarbons. Additionally, n-butane,isobutane, propane and liquid petroleum gas (LPG) are solvents which arereadily available for commercial use.

It should be appreciated that mixtures may include small amounts ofethane or higher hydrocarbons without departing from the scope andpurpose of the present invention. Further, butane in the form of n-(normal) or I- (iso) may be a constituent of fuel mixtures contemplatedby the present invention and frequently amounts of both n-butane andisobutane are at least trace components of such mixtures.

Vapor liquid equilibrium calculations may be performed for solutions ofmethane with these three solvents over a range of temperatures and molefractions of methane. Solutions with mole fractions of methane less than50 percent exhibit properties more like the solvent. Therefore,solutions with mole fractions of methane between 50 and 80 percent aremost relevant to the present invention. However, it should beappreciated that higher methane content is possible, but at the cost ofhaving to achieve and maintain lower temperatures.

The temperature range of interest is that which represents normalconditions. Thus, the upper end of relevant temperature range is thetemperature that can be maintained on a hot day. An arbitrarytemperature of 38° C. was chosen for the upper end of the temperaturerange.

The lower end of the temperature range represents a low temperaturewhich can be achieved without cryogenic cooling. For purposes ofanalysis, -1° C. was selected as the lower end of the temperature range.This low temperature allows the avoidance of special materials for tanksand pipes and permits the use of normal cooling equipment which employsfreon or freon substitutes as refrigerants.

FIGS. 1 through 3 show the bubble conditions of various mole fractionsof methane in butane, propane and LPG. The bubble pressures of all thesolutions are seen to increase with a corresponding increase in the molepercentage of methane.

As the mole fraction of methane is increased, the vapor pressure of thesolution increases. Therefore, a higher pressure is required to offsetthe higher vapor pressure of the solution.

Solution Energy Densities

Table A below presents a summary of the energy densities of a number ofpure hydrocarbon liquids and gases. Because hydrocarbon mixtures havinghigh propane content are used extensively for vehicle transportationfuels, it is interesting to compare the conditions at which the energydensities of these pure hydrocarbons equal or exceed the energy densityof propane.

Because natural gas is principally methane, the gross heating value9,474 MJ/m³ of methane at 38° C. and 24 MPa is representative ofcompressed natural gas (CNG) vehicle fuel. A drawback to the use of CNGas a vehicle fuel is this relatively low energy density compare togasoline, which generally has an energy density of 31,600 to 37,200MJ/m³, depending on the gasoline composition. Thus, the energy densityof CNG at 24 MPa generally is less than 30% of gasoline energydensities.

                  TABLE A    ______________________________________    Energy Densities of Pure Hydrocarbon Fluids                      Temp     Pressure                                     Gross Heating    Fuel       Phase  ° C.                               MPa   Value, MJ/m.sup.3    ______________________________________    methane    liquid -162     0.1   23,574    ethane     liquid -73      0.1   23,577    propane    liquid 38       1.4   23,580    isobutane  liquid 38       0.5   26,313    n-butane   liquid 38       0.5   27,492    isopentane liquid 38       0.2   29,216    n-pentane  liquid 38       0.2   29,371    n-hexane   liquid 38       0.1   31,174    n-heptane  liquid 38       0.1   32,052    n-octane   liquid 38       0.1   32,891    methane    gas    38       0.1   38    methane    gas    38       24    9,474    methane    gas    38       259   23,580    ethane     gas    38       0.1   66    ethane     gas    38       46    23,580    ______________________________________

Commercial propane mixtures, which are principally propane, have beenused as vehicle fuels because net energy costs are lower than gasoline.The energy densities of commercial propane mixtures are generally 65-75%of the energy densities of gasolines, so that with 35% larger fuel tanksthe propane fuel vehicle range is the same as for gasoline fuel.

It should be apparent from the above background information that it isdesirable to improve the energy density of natural gas beyond the energydensity of CNG at 24 MPa, i.e. beyond 9,474 MJ/m³. It is even moredesirable to improve the energy density of natural gas into the range ofpropane energy densities, i.e. into the range of 23,580 MJ/m³.

With reference to FIGS. 4 through 6, shown therein is the density ofeach solution plotted against the bubble temperature with the differentmole percentages of methane as a parameter. It should be noted thatthese mixtures are also at the bubble pressure corresponding to theindividual mixture-temperature and composition conditions.

From inspection of FIGS. 4 through 6, it is apparent that the solutiondensities decrease in two ways: (1) by increasing the methanepercentage, and (2) by increasing the temperature. Of course, thetendency of higher molar percentages of methane to reduce the mixturedensity is due to the lower density of methane. As more methane is addedto the mixture, the volumetric contribution of methane increases and thevolumes of the other, more dense components decrease.

The temperature effect on the solution densities is due to the increasedenergy of the mixture. Like water, a hydrocarbon solution that absorbsheat experiences some density decrease as the internal energy of themixture increases.

Standard Volume of Methane Stored

Referring to FIGS. 7 through 9, shown therein are plots of the amount ofmethane stored in the different solutions compared with the volume ofmethane in one liter of CNG at 21 MPa and 15° C. It should be noted thatthere are two parameters of interest in FIGS. 7 through 9: thecomposition of the solutions and the bubble temperature at which thecomparison was made.

The molar composition of the solutions is incremented by 5 percentmethane from the base value of 50 percent to the highest value for theparticular methane-solvent mixture. The leftmost point in each of thecompositional series corresponds to a temperature of -1° C. Moving tothe right in the direction of increasing bubble pressure, each lineencountered indicates a temperature increase of 6° C. until the upperend of 38° C. is reached.

FIG. 7 is the plot for the methane-butane mixture. The range of thenormalized volume of methane per liter of solution spans from 0.52 atabout 11 MPa, 38° C. and 50 mole-percent methane to about 0.94 at 15MPa, -1° C. and 80 percent methane.

The compositional dependence of the volume of methane stored can be seenby examining the curves of constant temperature. These curves aresubstantially linear, i.e. they have slopes which increase at a constantrate over the range of bubble pressures.

The influence of temperature on the relative volume of methane storedcan also be determined from FIGS. 7 through 9. By inspection of thecurves of constant composition, the volume of stored methane decreasesas the bubble temperature increases.

For 50 to 60 mole percent methane, this effect is substantially linearand roughly constant for all three compositions. However, the highermole fraction curves begin to depart from linearity and drop faster atthe higher temperature ends of the curves. Thus, the relative amount ofmethane stored becomes very sensitive to small perturbations in thebubble temperature. This behavior is similar to the shapes of the bubblecurves depicted in FIGS. 1 through 3.

The significant drop in the volume of methane stored as the pressure andtemperature increase delineates a favorable operating region inselecting a system of methane with butane, propane or LPG for use as amotor vehicle fuel. Since it is desirable to increase the stored methanein the liquid solutions, operational conditions should be chosen tocoincide with the predictable linear portions of the curves shown inFIGS. 7 through 9.

Referring to FIG. 8, shown therein is the normalized storage of methanefor the methane-propane solution. For propane, the minimum value of thenormalized storage volume for propane is 0.45 at 10 MPa and 37° C.; themaximum value is about 0.77 at 11 MPa and -1° C.

With reference to FIG. 9, shown therein is the normalized storage ofmethane for the methane-LPG mixture. For LPG, the minimum stored amountis 0.50 at 11 MPa and 37° C.; the maximum value is, 0.87 at 13 MPa and-1° C.

Similarities between the methane-butane plot of FIG. 7 and themethane-propane and methane-LPG graphs of FIGS. 8 and 9, respectively,are readily apparent. The curves of constant temperature and varyingmole fraction increase linearly as the mole fraction of methaneincreases. Further, the slopes of the curves decrease with eachtemperature increment. However, both the propane and LPG solutions showa faster drop than the butane mixture in the relative volume of methanestored along the constant composition curves.

From FIGS. 7 through 9, one can determine the amount of methane whichcan be stored in these solutions. For example, FIG. 9 shows that at 16°C. and 12 MPa, a 63-37 mole percent methane-LPG mixture holds about 68percent of the volume in CNG at 21 MPa and 15° C. This corresponds to areduction of 42 percent in the pressure which must be maintained. If thetemperature of the same composition were lowered to -1° C., the storedamount of methane would increase to 75 percent of CNG (21 MPa and 15°C.). The reduction in pressure would then be nearly 50 percent of thatfor CNG.

Comparative Energy Densities

In order to compare the energy densities of the methane solutions, theenergy densities may be normalized to gasoline having 36,400 MJ/m³ andplotted against the bubble temperature. The pressures are thosecorresponding to the bubble conditions given in FIGS. 1 through 3 forthe methane mixtures of interest.

With reference to FIG. 10, shown therein is the relative energy densityof the methane-butane solutions and CNG (21 MPa and 15° C.) versusbubble temperature. At -1° C., the mixture energy densities vary from 65percent to 45 percent of gasoline (36,400 MJ/m³) for 50 mole percentmethane and 80 mole percent methane, respectively. Note that the CNG (21MPa and 15° C.) provides about one-third the energy density of gasoline(36,400 MJ/m³).

Referring to FIG. 11, shown therein is the relative energy density ofthe methane-propane solutions versus the bubble temperatures. Withsimilarity to the butane-based solutions, the energy densities of thepropane-based solutions decrease with an increase in the temperature andwith an increase in the mole fraction of the methane stored.

At -1° C., the 50 mole percent methane-propane solution has 57 percentof the energy density of gasoline (36,400 MJ/m³). Also at -1° C., the 70mole percent methane mixture with propane has an energy density which is43 percent of that of the gasoline.

With reference to FIG. 12, shown therein is the energy density of themethane-LPG solutions relative to gasoline versus the bubbletemperatures. As one would expect, the energy densities of the LPG-basedsolutions decrease with an increase in temperature and with an increasein the mole fraction of the methane stored.

At -1° C., the 50 mole percent methane-LPG solution has 63 percent ofthe energy density of gasoline (36,400 MJ/m³). Also at -1° C., the 80mole percent methane mixture with LPG has an energy density which is 41percent of that in gasoline.

It is interesting to ascertain the energy contribution of methane forthe various mole fraction of methane in solution with butane, propaneand LPG. This information is presented in the following table.

    ______________________________________              Butane       Propane  LPG    Methane   Methane      Methane  Methane    Mole      Energy       Energy   Energy    Fraction  % Contrib    % Contrib                                    % Contrib    ______________________________________    50        24           29       25    55        27           33       29    60        32           38       34    65        36           43       38    70        42           48       44    75        48           past c.p.                                    50    80        55           past c.p.                                    57    ______________________________________

It can be seen from the above disclosure that storage of methane lighthydrocarbon liquids offers a solution to the high pressure storagerequirements of CNG. Over the temperature range of -1° C., the pressuresneeded for the methane-butane, methane-propane and methane-LPG mixtureswere all at least 8 MPa less than that of CNG at 21 MPa and 15° C.

Between -1° C. and 38° C., the energy densities of the methane-propane,methane-propane and methane-LPG mixtures exceed the energy density ofCNG. A 50 mole percent methane-butane mixture at -1° C., energy densityat 67 percent that of gasoline, appears to offer the greatest energydensity of the mixtures analyzed. This amount is more than double theenergy density of CNG at 21 MPa, but requires less than half thepressure at about 9 MPa.

Storage in a Motor Vehicle Tank

With reference to FIGS. 13 and 14, shown therein and designated byreference numeral 10 is a fuel storage system constructed in accordancewith the present invention. The fuel storage system 10 includes astorage tank 12, a layer of insulation 14 surrounding the storage tank12, a fuel line 16 from the storage tank 12 to the fuel injection system18 of the vehicle, an expansion valve 20, coolant lines 22 between thestorage tank 12 and the AC system 24 of the vehicle, a heat exchanger 26located within the storage tank 12, and a circulation pump 28.

It should be appreciated that the storage tank 12 has a suitable orifice(not shown) for filling the storage tank 12 with fuel 30 comprising anyof the methane and light hydrocarbons solutions, which are disclosed indetail hereinabove. Accordingly, the fuel 30 may be in the approximaterange of a 50 mole percent to 80 mole percent methane solution inn-butane, a 50 mole percent to 70 mole percent methane solution inpropane, or a 50 mole percent to 80 mole percent methane solution inLPG.

The storage tank 12 may be a 20-gallon CNG-type cylinder with a pressurerating of 21 MPa. The pressure rating of this cylinder is high enoughfor the working pressures of the methane solutions described hereinaboveand high enough to store CNG, if desired. Providing a 21 MPa rated tankallows alternative use of CNG if the methane solutions are notavailable.

Two vessel materials used in commercially available CNG cylinders arealuminum and a carbon composite. These materials are used singly or incombination for vessel construction. Thus, a range of expected heattransfer may be predicted from the heat transfer associated with thesematerials.

Further, the insulation 14 may be any type known in the art, such as afiberglass type similar to that found in home construction. It isreasonable to assume that the insulation 14 is similar to rock wool inits thermal conductivity.

For purposes of evaluation, the storage tank 12 is assumed to be in anisothermal environment at a temperature of 38° C. and out of directsunlight. The 38° C. temperature is selected arbitrarily as beingrepresentative of high temperatures in the United States during summermonths. Assuming the storage tank 12 to be out of direct sunlight isnecessary to avoid the addition of heat through solar adsorptivity.

The methane solution fuel inside the storage tank 12 is assumed to be at0° C. This assumption provides the greatest temperature differencebetween the methane solution fuel and the ambient conditions. Thus, thetemperature differential between ambient and the fuel is assumed to be38° C.

The amount of heat transferred to the fuel 30 from its surroundings maybe calculated according to the following equation: ##EQU2## where q isthe amount of heat transferred

.increment.T is the difference in ambient and fuel temperatures

R is the resistance to heat transfer.

For purposes of evaluation, the storage tank 12 is assumed to be ahollow cylinder with hemispherical ends. In calculating heat transfer,it is assumed that the heat would be transferred radially across boththe body and the ends of the storage tank 12.

The resistances necessary for computing the heat transfer to the fuel 30are the convective resistances inside and outside the storage tank 12and the conductive resistance across the insulation 14 and the storagetank wall. The convective resistances may be calculated from thefollowing equation: ##EQU3## where h is the convective heat transfercoefficient

A is the area exposed to convection

j denotes the inside or outside area of heat transfer

The resistance across the insulation 14 and the storage tank walls wasdetermined by summing the resistances of a hollow sphere and theresistance of a hollow cylinder. The resulting expression is: ##EQU4##where D is the diameter of the sphere and the cylinder

k is the thermal conductivity of the material

subscripts o and i represent the outside and the inside of theconducting medium, respectively

L is the length of the cylinder

The dimensions of the storage tank 12 and the pertinent physicalproperties of the materials are as follows:

    ______________________________________    Tank Dimensions    Outer Diameter      0.35 meters    Inner Diameter      0.31 meters    Cylinder Length     0.85 meters    Volume             0.074 cubic meters    Thermal Conductivity, k           Aluminum      122 kW/mk           Composite    0.36 kW/mk           Insulation   0.04 kW/mk    Convective Heat Transfer Coefficients, h    Ambient Air          0.055 kW/m.sup.2 k    Light Hydrocarbons   0.284 kW/m.sup.2 k    ______________________________________

These data and the above equations were used to determine the amount ofheat leak from ambient air at 32° C. to a liquid fuel at 0° C. as afunction of the thickness of the insulation 14. The results of thesecalculations are illustrated in FIG. 15.

The aluminum tank, having a much higher thermal conductivity than thecomposite tank, shows greater heat leakage for insulation thicknessbelow one inch. The difference between the two curves quickly decreaseswith increased insulation thickness.

It should be noted that the curves approach the same value at about oneinch of insulation. This convergence indicates that the insulation 14has become the controlling resistance in the heat transfer to the fuel30.

Further, it should be observed that the curves become asymptotic to avalue of about 950 kJ/hr at one inch of insulation. This characteristicimplies that insulation thicker than one inch would not aid greatly inthe resistance to heat leakage. Since it is desirable to provide a tankwith minimal outer dimensions, the thickness of the insulation 14 shouldnot exceed approximately one inch.

FIG. 15 suggests that the heat leakage to the storage tank 12 with oneinch of insulation 14 is about 950 kJ/hr. The addition of this energy tothe fuel storage system 10 containing the methane solution at its bubblepoint would raise the temperature of the fuel 30 and could result in avapor enrichment of methane and an increase in the system pressure.

Two techniques of heat removal may be utilized, singly or incombination, to control the temperature of the methane solutions in thestorage tank 12. The two techniques are: (1) using the vehicle'sexisting air-conditioning (AC) system; and (2) flashing the liquid fuelin an expansion valve.

Use of Vehicle's AC System

It should be appreciated that many feasible fuel mixtures would requirenot cooling. Only applications which make use of a sub-ambient storagetemperature require consideration of cooling and the consideration ofheat leakage.

A typical automobile AC system provides about 19,000 kJ/hr of coolingcapacity. For an ambient temperature of 38° C., the cooling load imposedon the typical AC system ranges from about 11,600 kJ/hr to 27,400 kJ/hr.The first value is the load associated with the steady-state operationat a vehicle speed of 30 miles per hour. The second value is the coolingload which occurs in the first ten minutes of operation following a hotsoak, i.e. ambient and solar heat saturation.

For the steady-state case, the required removal of 950 kJ/hr from thefuel is only about eight percent of the steady state cooling load. Inthe hot soak case, the 950 kJ/hr requirement increases the hot soakcooling load by only about three percent. Thus, existing AC systems maybe used to provide the required removal of 950 kJ/hr from the methanesolution fuels.

As shown in FIGS. 13 and 14, the AC cooling is provided to the methanesolution fuel by the coolant lines 22, the heat exchanger 26 and thecirculation pump 28. The coolant lines 22 supply AC coolant fluid to theheat exchanger 26.

The circulation pump 28 enhances the performance of the fuel storagesystem 10 in two ways. First, the circulation pump 28 circulates themethane solution fuel 30 around the heat exchanger 26 to increase thecontact of the fuel 30 with the heat exchanger 26. Secondly, thecirculation of the fuel 30 by the pump 28 keeps the contents of thestorage tank 12 well mixed.

Flash Expansion

Flash expansion involves using the fuel 30 itself to provide cooling tothe storage tank 12. Saturated liquid fuel 30 is withdrawn from thestorage tank 12 and allowed to expand inside the storage tank 12.

In order to provide the fuel storage system 10 with flash expansion, theexpansion valve 20 is located within the storage tank 12. The methanesolution fuel 30 is withdrawn from the storage tank 12 through theexpansion valve 20. The flashing fuel 30 cools as it expands across theexpansion valve 20 and draws heat from the bulk fuel 30 remaining in thestorage tank 12. The amount of cooling provided by flash expansion is afunction of the mass flow rate of the fuel 30 to the engine of thevehicle and the pressure on the discharge side of the expansion valve20.

It should be noted that the fuel pressure must be lowered before thefuel is introduced to the engine of the vehicle. Thus, flash expansionnot only provides cooling to the bulk fuel 30 in the storage tank 12 butalso performs the necessary function of lowering fuel pressure forintroduction to the engine.

In order to estimate the amount of cooling which may be provided byflash expansion, the amount of fuel to operate a vehicle must be known.The following table shows several commercially available cargo vans andassociated fuel consumption data. Cargo vans are selected as arepresentative vehicle for a fleet operation.

    ______________________________________                                    Engine Number                     Mpg     Mpg    size   of    Make   Model     (city)  (highway)                                    (liters)                                           Cylinders    ______________________________________    Chevrolet           G1500     15      19     4.3    6           G2500     14      18     5.0    8    Dodge  B1500     15      17     3.9    6           B2500     13      17     5.2    8    Ford   E150      13      17     4.9    6           Econoline 14      18     5.0    8    GMC    G1500     15      19     4.3    6           G2500     14      18     5.0    8    ______________________________________    Average Speed                 20 mph city   48 mph highway    Vehicle      6-cyl   6-cyl     8-cyl 8-cyl    Performance  (city)  (highway) (city)                                         (highway)    ______________________________________    Fuel Consumption                 1.3     2.5       1.4   2.7    (gallons gasoline    per hour)    Energy Consumption                 0.18    0.34      0.19  0.35    (MJ per hour)    ______________________________________

The energy consumption values from the above table may be converted toan equivalent mass of methane solutions from the energy densities,energy fractions and solution densities disclosed hereinabove. Thisconversion results in a mass flow rate of roughly 3.6 kg per hour forcity consumption of a 70 mole percent methane-butane mixture or a 65mole percent methane-propane solution.

Using the mass flow rate of 3.6 kg per hour, the cooling effect of flashexpansion can be approximated. Assuming that the fuel is fed to theexpansion valve 20 at 0° C. and the respective saturation pressure, theduty of the expansion valve 20 is illustrated in FIG. 15. The 70 molepercent methane in butane solution is shown as a solid line and the 65mole percent methane in propane is shown as a dotted line.

FIG. 15 is useful in determining the amount of heat that can be removedfrom a fuel solution if the outlet pressure of the expansion valve 20 isspecified. For the 70 mole percent methane-butane mixture, expansion to1.4 MPa indicates that about 420 kJ/hr can be removed from the fuel byflash expansion.

The 420 kJ/hr cooling from flash expansion is less than the needed 950kJ/hr heat leak described hereinabove. Thus, for the 70 mole percentmethane-butane fuel solution, heat removal via flash expansion alone issmaller than the heat being leaked into the fuel storage system 10 andheat will accumulate in the storage tank 12. Accordingly, in the case of70 mole percent methane-butane, supplemental cooling from the AC systemof the vehicle is required to prevent heat accumulation.

As illustrated by FIG. 15, flash expansion of the fuel to 1.4 MPa of the60 mole percent methane in propane solution removes about 950 kJ/hr ofheat from bulk fuel 30 in the storage tank 12. Thus, flash expansion ofthe 60 mole percent methane-propane mixture requires no additionalcooling by the AC system while the vehicle is running.

It is apparent from the foregoing estimates that the methane fuelsolutions may be maintained at sub-ambient temperatures by flashexpansion and the vehicle AC system, particularly in the case of fleetoperations. Thus, a solution of methane in light hydrocarbons is aviable vehicular fuel and has advantages in energy density and storagerequirements over CNG and LNG.

Changes may be made in the combinations, operations and arrangements ofthe various parts and elements described herein without departing fromthe spirit and scope of the invention as defined in the followingclaims.

What is claimed is:
 1. A method for storing a solution of methane and atleast one light hydrocarbon, the steps of the methodcomprising:introducing a solution produced by dissolution of gaseousmethane into at least one light hydrocarbon into a storage tank, thesolution comprising a mole percent of methane from about 50 to about 80percent; and maintaining the solution of methane and at least one lighthydrocarbon at a temperature of between about -1° C. and about 38° C.and at a pressure of between about 9.5 MPa and about 14.5 MPa such thatthe solution has an energy density of at least 11,000 MJ/m³.
 2. Themethod of claim 1 further comprising the step of:circulating thesolution of methane and at least one light hydrocarbon within thestorage tank.
 3. A method for storing a solution of methane and at leastone light hydrocarbon, the steps of the method comprising:selecting atleast one light hydrocarbon from the group consisting of butane, propaneand liquified petroleum gas; introducing a solution produced bydissolution of gaseous methane into at least one light hydrocarbon intoa storage tank, the solution comprising a mole percent of methane fromabout 50 to about 80 percent; and maintaining the solution of methaneand at least one light hydrocarbon at a temperature of between about -1°C. and about 38° C. and at a pressure of between about 9.5 MPa and about14.5 MPa such that the solution has an energy density of at least 11,000MJ/m₃.
 4. The method of claim 3 further comprising the stepof:circulating the solution of methane and at least one lighthydrocarbon within the storage tank.
 5. A method for storing a solutionof methane and at least one light hydrocarbon, the steps of the methodcomprising:introducing a solution produced by dissolution of gaseousmethane into at least one light hydrocarbon into a storage tank atambient temperature, the solution comprising a mole percent of methanefrom about 50 to about 80 percent; and maintaining the solution ofmethane and at least one light hydrocarbon at a pressure of betweenabout 9.5 MPa and about 14.5 MPa such that the solution has an energydensity of at least 11,000 MJ/m₃.